Stanford Medicine scientists have chalked up a major advance in growing human brain cells outside of the human brain — and outside of laboratory glassware.
In a study to be published online Sept. 16 in Nature, the researchers succeeded in transplanting self-organizing bits of laboratory-grown human brain tissue called cortical organoids into mice specially bioengineered and bred so that almost all of their cerebral cortex was missing. (The cerebral cortex is the outermost "rind" of the brain, to which much of our higher-level functioning such as cognition, language, attention and decision-making is attributed.)
The resulting vastly enlarged cavity in the mice's brains proved to be a hospitable environment. The human tissue survived, thrived, grew — and developed working connections to the mice's brain and beyond to the spinal cord.
"These animal models offer a unique opportunity to study how disease-associated alterations in human brain circuitry manifest in an intact nervous system," said Sergiu Pasca , MD, the Kenneth T. Norris, Jr. Professor II of Psychiatry and Behavioral Sciences and a member of the Wu Tsai Neurosciences Institute.
The researchers were astonished to find, in these mice, an important nerve-cell type that hasn't been previously glimpsed in laboratory culture and has been seen only in autopsied human brains.
Pasca, who is the Bonnie Uytengsu and Family Director of the Stanford Brain Organogenesis Program, a CZ BioHub Investigator and a science fellow of the Hoover Institution at Stanford, is the study's senior author. Lead co-authorship is shared by postdoctoral scholar Konstantin Kaganovsky , PhD; assistant professor of psychiatry and behavioral sciences Kevin Kelley , MD, PhD; neurosurgery instructor Tilo Gschwind , PhD; and medical student Paul Mahari.
The burden of neurodevelopmental disorders
The new methodology should speed research into the underlying biological causes of schizophrenia, epilepsy, profound autism and cerebral palsy, Pasca said. "Neuroscientists will be able to learn much more about the causes and mechanisms of neurodevelopmental and pregnancy-incurred disorders and to test possible interventions to correct or prevent them."
Such conditions are exceedingly difficult to study at the molecular and cellular level because the brain is more complex than any other organ and because living human brain tissue is nearly always inaccessible.
One in 20 American adults is troubled by a severe psychiatric illness; the associated medical, social and economic burdens are huge. More than 1 in 100 adults suffers from schizophrenia, believed to be largely the result of brain-circuit abnormalities predating birth. Some 1% of all people remain epileptic throughout adulthood.
According to Alison Singer, president of the Autism Science Foundation, a patient-advocacy group that funds autism research, one in every 218 American children meets the criteria for profound autism: an unarguably disabled state requiring round-the-clock supervision to ensure safety, as well as for assistance with activities of daily living such as getting dressed, using the bathroom and preparing food.
"Profoundly autistic patients have pronounced cognitive disabilities, with measured IQs below 50. They are vulnerable to sleep disorders, epilepsy and self-injury; they are disproportionally likely to exhibit aggressive behavior," she said.
In their laboratory-generated "xenocortical" mice, the researchers were also able to show human cerebral nerve cells' singular vulnerability to oxygen deprivation — known to be a cause, when it occurs during pregnancy or around birth, of cerebral palsy, which affects 3 in 1,000 Americans and is a risk factor for epilepsy and autism.
"While animal models have been extremely helpful, some biological features seem to be uniquely human," Pasca said.
He and his colleagues have circumvented that constraint via a series of extraordinary laboratory leaps. More than a decade ago, they described the creation of what are now known as organoids, three-dimensional laboratory cultures resembling specific regions of the brain. They transformed human skin cells into stem cells that can differentiate into most of the body's cell types, then gently coaxed these cells to form self-organizing clusters of brain tissue whose internal architecture closely resembled that of circuitry found in the cerebral cortex.
"We can now generate a broad range of brain cell types, present throughout the developing human nervous system," Pasca said.
Various organoids mimicking distinct brain regions can be juxtaposed to spur interaction between them and interpenetration of their component nerve cells, or neurons, to form complex signaling circuits resembling those in the human brain. These fused multi-organoid combos , which Pasca later named assembloids, could be connected in threes or even fours to re-create pathways in our bodies that drive movement or sensation.
But there were limits. "We couldn't study complex human behavior in a dish," Pasca said. In addition, the absence of blood supply, immune-cell infiltration, connections to sensory input and motor output, and failure of some component cell types to fully mature put a cap on what could be learned from probing organoids or assembloids in culture to study neurodevelopmental disorders or human brain processes in general.
Might integrating human organoids into a living rodent's brain — particularly when that brain was still in an early stage of circuit formation — address these limitations?
From the dish to the rodent's brain
In 2022, Pasca and his colleagues transplanted human cortical organoids into newborn rats' brains and saw much better neuronal development than could be achieved in a dish. The implanted human organoids, measuring perhaps one-fifth of an inch in diameter when transplanted, occupied a full one-third of the hemisphere of the rat brain six months later. At eight months, individual neurons from the human organoids in the rat brains were substantially bigger and more electrically active than those in the organoids — generated the same way, at the same time — that remained in a dish instead of being transplanted. They exhibited much more sophisticated branching patterns. And they integrated with their rat-neuron neighbors to form working brain circuits.
"The cells we implant carry the genetic material of the person they're derived from — whether that person is a patient or a healthy individual — allowing us to study downstream disease effects in brain cells and circuits," Pasca said.
"The idea that you can make an organoid model with an individual's unique genetic character and use that to learn what's gone awry in that individual's brain is a critical step toward precision medicine," said Singer, the Autism Science Foundation president.
In that same 2022 study, Pasca's team seeded neonatal rats' cortical regions with brain tissue from young patients with a rare but devastating genetic condition called Timothy syndrome, characterized by severe autism and heightened vulnerability to epilepsy and schizophrenia. Pasca and his colleagues were able to pinpoint the disorder's molecular defect and, later on, found a candidate drug to treat it. That drug is now in preparation for entering phase 1/2 safety trials.
Even then, there were limits: For example, the rat neurons developed much more quickly than the human neurons did.
"The two parallel developing systems are in competition for turf," Pasca said.
The advent of apallial mice
In the new study, Pasca's team came up with a way to eliminate much of that competition. They produced a laboratory mouse strain that was genetically engineered so that the "starter cells" that give rise to the evolutionarily more-recent addition to the cerebral cortex — the neocortex — and certain other brain structures can't get formed.
Achieving this necessitated several technically challenging steps. But the team finally generated mice that survived in healthy condition, despite the absence of most of their cortex and hippocampus, a brain structure important for memory formation.
Upon reaching adulthood, the brains of the "apallial mice" (so named because the missing cortical tissue develops from a prenatal brain structure called the pallium) contained a mere 2% of ordinary mice's equivalent cortical content, leaving plenty of space that could have been occupied by human cortical cells.
"Our findings suggest that, when cortical circuitry is lost very early in development, the cortex may not be solely responsible for all of the functions traditionally attributed to it," Pasca said. "Other brain regions may compensate for some of the missing cortical circuitry as the brain develops."
While pretty much indistinguishable from normal mice at first glance, the apallial mice did exhibit minor but measurable behavioral quirks: a slightly more cautious gait, probably reflecting somewhat compromised fine-motor coordination, and a tendency to be more forgetful about recently encountered novel environments. Their behavioral performance also varied more from one individual to the next than normal control mice's did, suggesting that the cortex has a stabilizing role in brain function.
A more complete model
Newborn apallial mice served as recipients for human cortical organoids. About two months after developing cortical organoids (derived from healthy donors who had consented to having their cells transplanted into animals), the investigators surgically positioned organoids (typically more than one per recipient) containing perhaps 100,000 cells apiece in 2-day-old apallial animals' enlarged fluid-filled cavities in the brain.
A lopsided majority of these operations succeeded. The human tissue survived and thrived in cortex-depleted mice's brains. Three months after the surgical procedure, more than 90% by volume of the cortical tissue in the mice's brains was human. Integration of human and mouse neurons into circuits was proceeding apace.
Three to six months after the transplant surgery, the xenocortical mice's performance on behavioral tests was generally similar to that of their same-age normal peers.
In a proof-of-concepts experiment demonstrating how these mice can be used for studying disease, xenocortical mice were exposed to five hours of low oxygen. The exposure wreaked substantial damage on their human-originated cortical tissue. Oxygen-deprived xenocortical mice appeared to have trouble sustaining a steady gait and maintaining their balance, reminiscent of what is seen in children with cerebral palsy. In contrast, normal and apallial mice were virtually unaffected.
"Finding out what accounts for this difference could yield clues about human neural susceptibility to oxygen deprivation, shed light on mechanisms underlying cerebral palsy and provide a platform for testing potential therapeutic strategies," Pasca said.
Spotted: A seldom-glimpsed brain-cell type
Remarkably, the xenocortical mice's human brain tissue contained detectable numbers of what the researchers determined were von Economo neurons, or VENs. These nerve cells are peculiar in several ways: They've previously been observed only in postmortem brain tissue, haven't previously been generated in culture, and hadn't turned up in the earlier transplant experiments under more crowded conditions in recipient rodents' brains. They're very rare, accounting for roughly one in every 90,000 cortical neurons. Originally thought to be human-specific, they're now known to be present in other large-brained, highly social animals including great apes, elephants, dolphins and whales. In humans, they're situated in the fronto-insular cortex and anterior cingulate cortex, two brain regions involved in social awareness and decision-making. Their projections can extend well beyond these cortical centers. They're gigantic — much bigger than most other neurons — and distinctively cigar-shaped.
"This cell type appears to be particularly vulnerable in frontotemporal dementia, a neurodegenerative disorder that can begin in midlife," Pasca said. "In some forms of the disease, these cells are markedly reduced, and early symptoms can include changes in social behavior and personality or difficulties with language, often before prominent memory problems emerge."
VENs can't be made in laboratory culture. "Yet here they were, sitting in the xenocortical mice's human tissue. Now we can generate these rare cells from a healthy person and study them in a living, behaving animal to learn more about what they're doing. Or we can derive cells from patients with frontotemporal dementia — or other neurological conditions — transfer them into these mice, test the animals for cellular or circuit-level correlates of behavioral disability, and then screen therapeutic candidates."
Pasca stressed the importance of remaining within ethical boundaries drawn by reflection on this research: "Throughout several years of experimentation, we have received input from ethicists, neurobiologists with expertise in primate and human cortical biology, patient advocates, philosophers, and legal scholars."
In November 2025, Pasca organized a conference in Asilomar, California, to debate the ethical implications of using human stem cell models and their transplantation.
"An overriding argument questioned the ethics of not conducting this research in the face of the suffering of hundreds of millions of people afflicted with neurological disorders that today are uncurable but tomorrow could yield treatments we discover by using this model," he said.
Stanford University's Office of Technology Licensing holds patents for the generation of cortical organoids. Pasca is listed as an inventor. The office also holds a provisional patent application for transplantation of organoids, with Pasca, Kelley and three other study co-authors as inventors.
A researcher from the Basque Center on Cognition, Brain and Language in Spain contributed to the work.
The study was funded by the Stanford Wu Tsai Neuroscience Institute, the Kwan Funds, the Senkut Funds, and the Brain & Behavior Research Foundation.
